Abstract

Spectra of the N-phenyl-5,6-dihydro-2,4-diphenylbenzo[c,h]quinolinium tetrafluoroborate (1) and of the N-phenyl-5,6,8,9tetrahydro-7-phenyldibenzo[c,h] acridinium tetrafluoroborate (2) were recorded in various solvents and temperatures. The analysis of the 1H-NMR spectra of the tetrafluoroborate salt 1, recorded in acetone, acetonitrile, 1,1,2,2-tetrachloroethane and chloroform, revealed the existence of an equilibrium between two conformers in solution. Tight ion-pairing in chloroform led to a smaller barrier for interconversion between the two conformers. In more polar solvents, where the dihydrobenzoquinolinium exists as a free cation, theoretical calculations predicted larger barriers. The spectra of 1 in 1,1,2,2-tetrachloroethane also varied with temperature, resembling at highertemperatures the spectrum in CDCI3 and at 300K spectra in more polar media. Spectra of 2 did not vary with the solvent or the temperature, in an indication of a much higher barrier to conformational interconversion, because of a greater steric hindrance between the N-phenyl substituent and the dihydrobenzo rings. Copyright © 2009 John Wiley & Sons, Ltd.